Already known as non-volatile memories are flash memories, FeRAMs, MRAMs, phase-change memories. U.S. Pat. No. 6,172,902, for example, discloses an MRAM incorporated in a membrane, and U.S. Pat. No. 5,166,758 discloses the construction of a phase-change memory.
Since higher densities are required of memories for use in portable information terminals and the like, attention has been directed to phase-change non-volatile memories, and various improvements have been made in such memories [WO97/05665 (Japanese Unexamined Patent Publication No. 1999-510317), WO98/19350 (Japanese Unexamined Patent Publication No. 2001-502848), WO99/54128 (Japanese Unexamined Patent Publication No. 2002-512439, U.S. Pat. Nos. 6,339,544, 5,536,947, etc.).
For example, WO98/336446 (Japanese Unexamined Patent Publication No. 2001-504279) discloses, as shown in FIG. 11, a phase-change non-volatile memory comprising a lower electrode 51, an upper electrode 52, and a phase-change material layer 53 which is formed therebetween and through which current can be passed via the electrodes 51, 52. The phase-change material layer 53 comprises a chalcogenide material which is reversibly changeable in phase between an amorphous (noncrystalline) state of high resistance and a crystalline state of low resistance. The material is changed to the noncrystalline state or crystalline state by the passage of current to control the resistance value. For example when data is stored (written), the phase-change material layer 53 is changed from the amorphous state to the crystalline state and thereby given a low resistance value, while when data is to be erased, the layer 53 is changed from the crystalline state to the amorphous state and given a high resistance value. Thus the difference in resistance value is read to use the layer as a memory.
In the construction shown in FIG. 11, a joint portion 51a between the lower electrode 51 and the phase-change material layer 53 is shaped in a frustoconical form to thereby provide an improved current density. The joint portion 51a is formed by undercutting a pattern (not shown) formed on the phase-change material layer 53 by photolithography. After the pattern is removed, the layer 53 is formed on the joint portion 51a by photolithography.
WO97/40499 (Japanese Unexamined Patent Publication No. 2000-509204) also discloses a structure which is similar to the above and in which a joint portion between a lower electrode and a phase-change material layer is tapered toward the phase-change material layer with a decreasing cross sectional area to give a higher current density at the tapered end.
Giving an increased current density in this way is effective from the viewpoint-of achieving savings in the electric power of the memory for writing and erasing data. However, if the contact between the electrode and the phase-change material layer is diminished, there arises the problem that faulty conduction is liable to occur between the electrode and the layer to result in a lower yield. The need to form the phase-change material layer accurately at the position where the joint portion 51a is formed further entails the problem of reducing the freedom of design.